The field of application of the invention is that of a method for indicating a combination of signals to be applied to a physical element in order to obtain a change in the state of a given signal following a change in the state of another signal on which the given signal depends.
2. Description of the Related Art
In complex physical elements, such as all or part of high-density integrated circuits, signals generated by these physical elements can depend on many other signals. It is therefore difficult to know if the physical element is behaving correctly.
Before etching into the silicon of electric circuits, the bad circuits must be detected in order to avoid using them. The technique used to sort the good circuits from the bad circuits is one that consists of applying stimuli to the circuit and comparing their responses to precalculated values.
In the case of a purely combinational circuit with two hundred inputs, it would be necessary to apply two to the power two hundred combinations of binary states in the inputs in order to test all the possible cases. A testing device operating at 50 MHz would take a thousand years to test a single circuit. These figures increase exponentially when there are stored states in the circuit.
It is therefore necessary to minimize the number of combinations of binary states to be applied to signals of the circuit in order to find the possible faults in the circuit.
For a first signal, normally generated by a physical element as a function of other signals applied to this physical element it is known from U.S. Pat. Nos. 5,434,794 and 5,737,242 to use binary decision diagrams to determine possible binary states of the first signal resulting from combinations of binary states of a second signal with binary states of third signals of the circuit.
The advantage of binary decision diagrams is that it is possible to represent in a computer memory, in a particularly compact way, the combinations of binary states of signals that lead to a given binary state of the first signal.
However, the binary decision diagrams of the prior art provide a static image of possible combinations of binary states of a second signal with third signals that produce a given binary state of the first signal. It is therefore possible for a combination of a first binary state of the second signal with binary states of third signals to produce the same given binary state of the first signal as a combination of a second binary state of the second signal with other binary states of third signals. Furthermore, it is possible to observe in the physical element either the given binary state of the first signal for the first binary state or the second binary state of the second signal, whereas the latter results not from binary combinations for which the physical element is designed, but from faults, such as a signal sticking in the physical element. Signal sticking means that a signal is in a state not intended by design, for example a state that results from a short circuit or from damage.
In order to ensure that the state of a first signal actually results from the state of a second signal, it is important to observe whether a state transition of the second signal induces a state transition of the first signal, the states of the other signals remaining unchanged. If, moreover, the transition of the first signal is does not match the transition expected, this may be interpreted as an error.